Sometimes, you may want to store the result of a
condition, being true or false, in a variable. To do this, you can
declare a variable that would hold a Boolean value. Here is an example of declaring a Boolean variable when
the form opens:
Dim IsMarried
When a Boolean variable has been declared, you can assign it
a True or a False value.
To convert a value or an expression to Boolean, use the CBool()
function.
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The If...Then...Else Statement
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The If...Then statement offers only one
alternative: to act if the condition is true. Whenever you would like to
apply an alternative in case the condition is false, you can use
the If...Then...Else statement. The
formula of this statement is:
If ConditionToCheck is True Then Statement1 Else Statement2 End If
When this section of code executes, if the ConditionToCheck
is true, then the first statement, Statement1, is executed. If
the ConditionToCheck is false, the second statement, in this case Statement2,
is executed.
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Practical Learning: Using If...Then...Else |
- To use If...Then...Else, change the code as follows:
<%@ Language="VBScript" %> <html> <head> <title>CD Publisher</title> </head> <body> <h1>CD Publisher</h1> <form method="GET" action="cdpublisher2.asp"> <table border="0" width="553"> <tr> <td width="191">Number of CDs Ordered:</td> <td width="113"><input type="text" name="txtQuantity" size="10" value=<% =Request.QueryString("txtQuantity") %> > </td> <td width="67"><input type="submit" value="Calculate" name="btnCalculate"></td> <td width="162"><input type="reset" value="Start New Order" name="btnReset"></td> </tr> </table> </form> <% Dim Quantity Dim UnitPrice Dim TotalPrice Dim strCD Quantity = CInt(Request.QueryString("txtQuantity")) strCD = "CDs" If Quantity = 1 Then strCD = "CD" Response.Write("<p>Since you ordered " & Quantity & " " & strCD & ",<br>") If Quantity < 20 Then UnitPrice = 20 Else UnitPrice = 6.25 End If Response.Write("Each CD will cost: " & UnitPrice & "<br>") TotalPrice = Quantity * UnitPrice Response.Write("And the total price is: " & TotalPrice & "</p>") %> </body> </html> - Save the file
- Return to the browser, refresh it, enter a new value in the Quantity text box and click Calculate
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The If...Then...ElseIf Statement
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The If...Then...ElseIf
statement acts like the If...Then...Else expression, except that it
offers as many choices as necessary. The formula to use is:
If Condition1 is True Then Statement1 ElseIf Condition2 is True Then Statement2 ElseIf Conditionk is True Then Statementk End If
The program will first examine Condition1. If Condition1
is true, the program will execute Statment1 and stop examining
conditions. If Condition1 is false, the program will examine Condition2
and act accordingly. Whenever a condition is false, the program will
continue examining the conditions until it finds one. Once a true
condition has been found and its statement executed, the program will
terminate the conditional examination at End If.
There is still a possibility that none of the stated
conditions is true. In this case, you should provide a "catch
all" condition. This is done with a last Else section. The Else
section must be the last in the list of conditions and would act if none
of the primary conditions is true. The formula to use would be:
If Condition1 is True Then Statement1 ElseIf Condition2 is True Then Statement2 ElseIf Conditionk is True Then Statementk Else CatchAllStatement End If |
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Practical Learning: Using If...Then...ElseIf |
- To use If...Then...Else, change the code as follows:
<%@ Language="VBScript" %> <html> <head> <title>CD Publisher</title> </head> <body> <h1>CD Publisher</h1> <form method="GET" action="cdpublisher2.asp"> <table border="0" width="553"> <tr> <td width="191">Number of CDs Ordered:</td> <td width="113"><input type="text" name="txtQuantity" size="10" value=<% =Request.QueryString("txtQuantity") %> > </td> <td width="67"><input type="submit" value="Calculate" name="btnCalculate"></td> <td width="162"><input type="reset" value="Start New Order" name="btnReset"></td> </tr> </table> </form> <% Dim Quantity Dim UnitPrice Dim TotalPrice Dim strCD Quantity = CInt(Request.QueryString("txtQuantity")) strCD = "CDs" If Quantity = 1 Then strCD = "CD" Response.Write("<p>Since you ordered " & Quantity & " " & strCD & ",<br>") ' The price of one CD will depend on the number ordered ' The more the customer orders, the lower value each If Quantity < 20 Then UnitPrice = 20 ElseIf Quantity < 50 Then UnitPrice = 15 ElseIf Quantity < 100 Then UnitPrice = 12 ElseIf Quantity < 500 Then UnitPrice = 8 Else UnitPrice = 5 End If Response.Write("Each CD will cost: " & UnitPrice & "<br>") TotalPrice = Quantity * UnitPrice Response.Write("And the total price is: " & TotalPrice & "</p>") %> </body> </html> - Save the file
- Return to the browser, refresh it, enter a new value in the Quantity text
box and click Calculate
- Return to your text editor
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The Select Case Statement
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If you have a large number of conditions to examine,
the If...Then...Else will go through each one of them. Microsoft
Visual Basic
offers the alternative of jumping to the statement that applies to the
state of the condition.
The formula of the Select Case is:
Select Case Expression Case Expression1 Statement1 Case Expression2 Statement2 Case Expressionk Statementk End Select
The Expression will be examined and evaluated
once. Then Microsoft Visual Basic will compare the result of this examination with the Expression
of each case. Once it finds one that matches, it would execute the
corresponding Statement.
If you anticipate that there could be no match between
the Expression and one of the Expressions, you can use a Case
Else statement at the end of the list. The formula to use would
be:
Select Case Expression
Case Expression1
Statement1
Case Expression2
Statement2
Case Expressionk
Statementk
Case Else
Statementk
End Select
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Loops Repeaters
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Introduction
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A loop is an expression used to repeat an action.
Microsoft Visual Basic presents many variations of the loops and they
combine the Do and the Loop keywords.
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The Do...While Loop
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The formula of the Do... While loop is:
Do While Condition Statement(s) Loop
This expression examines the Condition. If the
condition is true, then it executes the Statement or statements.
After executing the statement(s), it goes back to examine the Condition.
AS LONG AS the Condition is true, the Statement will be
executed and the Condition will be tested again. If the Condition
is false or once the condition becomes false, the statement will not be
executed and the the program will move on. As you may guess already, the Condition
must provide a way for it to be true and to be false.
Here is an example:
<%@ Language="VBScript" %>
<html>
<head>
<title>Active Server Pages Tutorials</title>
</head>
<body>
<%
Dim Counter
Counter = 0
Do While Counter <= 12
Response.Write(Counter & " ")
Counter = Counter + 1
Loop
%>
</body>
</html>
This would produce:
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The Do...Loop...While Statement
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Since the Do...While statement tests the Condition
first before executing the Statement, sometimes you will want the
program to execute the Statement first, then go back and test the Condition.
Microsoft Visual Basic offers a reverse to the formula as follows:
Do Statement(s) Loop While Condition
In this case, the Statement or Statements
will be executed first. Then the Condition will be tested. If the Condition
is true, the program will execute the Statement again. The program
will continue this examination-execution as long as the Condition
is true. The big difference here is that even if the Condition is
false, the program will have executed the Condition at least once.
Here is an example:
<%@ Language="VBScript" %>
<html>
<head>
<title>Active Server Pages Tutorials</title>
</head>
<body>
<%
Dim Counter
Counter = 2.25
Do
Response.Write(Counter & "<br>")
Counter = Counter + 0.35
Loop While Counter <= 6
%>
</body>
</html>
This would produce:
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The Do...Until...Loop Statement
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An alternative to the Do...While loop is the Do...Until
loop. Its formula is:
Do Until ConditionStatement(s) Loop
This loop will first examine the Condition,
instead of examining whether the Condition is true, it will test
whether the Condition is false.
Here is an example:
<%@ Language="VBScript" %>
<html>
<head>
<title>Active Server Pages Tutorials</title>
</head>
<body>
<%
Dim InterestRate
InterestRate = 7.25
Do Until InterestRate = 16
Response.Write("Interest Rate: " & InterestRate & "%<br>")
InterestRate = InterestRate + 0.25
Loop
%>
</body>
</html>
This would produce:
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The Do...Loop...Until Statement
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An alternative to the Do...Until...loop
consists of executing the the Statement first. The formula used is:
Do Statement(s) Loop Until Condition
This expression executes the Statement first.
After executing the Statement, it examines the Condition. If
the Condition is False, then it goes back and executes the Statement
again and re-check the Condition. Once the Condition becomes
true, it would stop and move on; but as long as the Condition is
False, the Statement would be executed.
Here is an example:
<%@ Language="VBScript" %>
<html>
<head>
<title>Active Server Pages Tutorials</title>
</head>
<body>
<%
Dim InterestRate
InterestRate = 7.25
Do
Response.Write("Interest Rate: " & InterestRate & "%<br>")
InterestRate = InterestRate + 0.25
Loop Until InterestRate = 16
%>
</body>
</html>
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Loop Counters
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Introduction
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The looping statements we reviewed above are used when
you don't know or can't anticipate the number of times a condition needs
to be checked in order to execute a statement. If you know with certainty
how many times you want to execute a statement, you can use another form
of loops that use the For...Next expression.
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The For...To...Next Loop
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One of the loop counters you can use is For...To...Next.
Its formula is:
For Counter = Start To End Statement(s) Next
Used for counting, the expression begins counting at
the Start point. Then it examines whether the current value (after
starting to count) is greater than End. If that's the case, it then
executes the Statement(s). Next, it increments the value of Counter
by 1 and examines the condition again. This process goes on until the
value of Counter becomes equal to the End value. Once this
condition is reached, the looping stops.
Here is an example:
<%@ Language="VBScript" %>
<html>
<head>
<title>Active Server Pages Tutorials</title>
</head>
<body>
<%
Dim Counter
For Counter = 0 to 26
Response.Write(Counter & " ")
Next
%>
</body>
</html>
This would produce:
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Stepping the Counting Loop
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The formula above will increment the counting by 1 at
the end of each statement. If you want to control how the incrementing
processes, you can set your own, using the Step option. Here is the
formula:
For Counter = Start To End Step Increment Statement(s) Next Counter
You can set the incrementing value to your choice. If
the value of Increment is positive, the Counter will be
added its value. This means that you can give it a negative value, in
which case the Counter will be subtracted the set value.
Here is an example:
<%@ Language="VBScript" %>
<html>
<head>
<title>Active Server Pages Tutorials</title>
</head>
<body>
<%
Dim Counter
For Counter = 0 to 46 Step 2
Response.Write(Counter & " ")
Next
%>
</body>
</html>
This would produce:
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For Each Item In the Loop
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Since the For...Next loop is used to execute a
group of statements based on the current result of the loop counting from Start
to End, an alternative is to state various steps in the loop and
execute a group of statements for each one of the elements in the group.
This is mostly used when dealing with a collection of items.
The formula is:
For Each Element In Group Statement(s) Next Element
The loop will execute the Statement(s) for each
Element in the Group.
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